The overall objective of this study is to determine the role of cellular immunity in fibroma tumor regression either as an anti-tumor cell function or an anti-virus function. Lymphocyte and macrophage cytotoxicity to fibroma tumor cells will be evaluated in adult rabbits with regressing tumors and new-born rabbits with progressive, growing tumors (Cr51 release test; microcytotoxicity test) to determine if newborn tumor-bearing rabbits have a defective cell-mediated immunity. Anti-tumor cell immunity of macrophages will be related to anti-viral immunity (ability to restrict virus replication; interferon production) in the adult and newborn rabbits. The mechanism and specificity of macrophage activation will be studied in vitro by evaluating the roles of T-cells and/or cytophilic antibodies on macrophage activation. These studies will be performed using fibroma virus, myxoma virus, and vaccinia virus. BIBLIOGRAPHIC REFERENCES: Schultz, R. M., Woan, M. C., and Tompkins, W.A.F. Macrophage immunity to vaccinia virus: factors affecting macrophage immunity in vitro. J. Reticuloendothelial. Soc. 16:37-47, 1974. Tompkins, W.A.F., Rama Rao, G.V., and Woan, M.C. Immune and nonimmune macrophage resistance to the fibroma-myxoma virus complex in rabbits. J. Reticuloendothelial. Soc. 18:23-33, 1975.